bpo-38361: syslog: fixed making default "ident" from sys.argv[0] (GH-16557)

The default value of "ident" parameter should be sys.argv[0] with leading path
components stripped, but it contained the last slash, i.e. '/program' instead
of 'program'.

BPO issue: https://bugs.python.org/issue38361


https://bugs.python.org/issue38361